Ultrasonic, flow measuring devices are widely used in process and automation technology. They allow, in a simple manner, contactless ascertaining of volume flow in a pipeline.
Known ultrasonic, flow measuring devices work frequently according to the Doppler principle or according to the travel-time difference principle.
In the case of the travel-time difference principle, the different travel times of ultrasonic pulses relative to the flow direction of the liquid are evaluated.
For this, the ultrasonic pulses are sent both in the direction of flow, as well as opposite to the direction of flow. From the travel-time difference, the flow velocity and, therewith, in the case of the known diameter of the pipeline section, the volume flow, e.g. volume flow rate, can be determined.
In the case of the Doppler principle, ultrasonic waves of a certain frequency are coupled into the liquid and the ultrasonic waves reflected from the liquid are evaluated. From the frequency shift between the injected and reflected waves, the flow velocity of the liquid can likewise be determined.
Reflections occur in the liquid, however, only if small air bubbles or impurities are present, so that this principle is mainly used for contaminated liquids.
The ultrasonic waves are generated and, respectively, received with the help of so-called ultrasonic transducers. For this, the ultrasonic transducer is placed securely on the pipe wall of the relevant section of pipeline. More recently, clamp-on ultrasonic flow measuring systems are available. With these systems, the ultrasonic transducers are only pressed against the tube wall with a clamp. Such systems are known e.g. from European Patent 686 255, U.S. Pat. Nos. 4,484,478 or 4,598,593.
Another type of ultrasonic, flow measuring device, which operates according to the travel-time difference principle, is described in U.S. Pat. No. 5,052,230. Here, the travel time is ascertained by means of bursts in the form of short, sinusoidal, ultrasonic pulses.
Ultrasonic transducers are typically composed of a piezoelectric element, also called a piezo for short, and a coupling element, also called a coupling wedge or, less frequently, a lead-in element, which is made of plastic. The ultrasonic waves are generated in the piezoelectric element and conveyed via the coupling element to the tube wall and from there into the liquid. Since sound velocities are different in liquids and plastics, the ultrasonic waves are refracted when passing from one medium to another. The angle of refraction is determined by Snell's law. Thus, the angle of refraction is dependent on the ratio of the propagation velocities in the two media.
From the state of the art, electrical contacting of piezoelectric elements is known to technically qualified persons. The electric contacts are located on the side of the piezoelement opposite to the flatly mounted side. The piezoelectric element has electrically conductive coatings on both sides and the coating on the fixedly mounted side extends to the opposite side of the piezoelectric element, where it can be contacted. This leads to the fact that the electrically conductive coating on the side opposite the flatly mounted side covers only part of the surface. Thus only a portion of the piezo surface is used.
Conventional coupling elements are manufactured of plastic and have a bore, ih which the piezoelectric element is applied. For manufacturing reasons, this application is burdened by tolerances. For instance, a non-uniform distribution of the adhesive can occur, whereby the piezoelectric element assumes an undefined spacing from the coupling element. Furthermore, the placement of the piezoelectric element in the bore of the coupling element has tolerance issues, such that the position of the area of sound emergence varies from sensor to sensor. The manufacture of a piezoelectric element and therewith the size of the area of sound emergence of the sensor is itself subject to certain tolerances. The sum of these tolerances results in measurement error.
Typically, a sensor holder is oriented on the pipe and correspondingly secured and the coupling element is oriented in the sensor holder. As a result, manufacturing related tolerances of individual, assembled components add to the error in the position of the piezo relative to the pipeline wall, or relative to the measured medium and/or relative to an additional sensor or its piezoelectric element of the measuring system. In order to keep these errors small, the individual components have to be formed by complicated and expensive processes.
As a result of the described construction, usually high mechanical stresses occur on the piezoelectric element. These stresses arise from thermal expansion or the mechanical attachment of the sensor to a pipeline and are transmitted via the walls of the coupling element.